1. Field of the Invention
The present invention relates to a device for generating light pulses that are separated in terms of time, having a light source that emits a sequence of light pulses.
2. The Prior Art
Such devices are used, for example, for ultra-fast time-resolved spectroscopy. In this connection, mode-coupled lasers are usually used as light sources. So-called pump-probe techniques are used for measuring and investigating the time progression of physical or chemical processes. Such techniques have led to significant advances in various scientific and technical sectors. Studies regarding relaxation dynamics in solids, liquids, and gases, time-resolved terahertz spectroscopy, and signal analysis in optical communications technology should be mentioned. Mode-coupled lasers are used as light pulse sources in synchrotron radiation sources in order to control the generation of electron bundles in terms of time. They are also used in order to analyze the time behavior of electron, UV-light, or X-ray pulses. All these applications have in common that it must be possible to precisely control the arrival times of the light pulses in an interaction center of the experiment, in each instance. In most cases, it must be possible to vary the arrival times, i.e. the time intervals between light pulses that arrive consecutively, in order to thereby be able to scan the time progression of the physical, technical, or chemical process to be studied.
It is known to generate consecutive light pulses having an adjustable time interval via a single light source. The light beam of the source is divided up into two partial beams and brought together again. A delay distance having a variable length is situated in one arm. In this method of procedure, the variable time interval between the light pulses results from the different running times in the arms of such an interferometer. The variable length is generally implemented by means of an electromechanically moved mirror. It is a disadvantage that the mirror movement is relatively slow, so that the time interval between the light pulses can be varied only in correspondingly slow manner. This requirement results in undesirably long scanning times. It is a further disadvantage that the mechanical mirror adjustment is susceptible to incorrect settings. Furthermore, the mirror movement brings about an undesirable variation in the beam diameter, caused by the divergence of the light beam.
To overcome the disadvantages indicated above, the so-called ASOPS technique (“asynchronous optical sampling”) has become known. In this connection, two light sources are used, which emit periodic sequences of light pulses, whereby the light pulse sequences are superimposed in the interaction center of the experiment, in each instance. The light pulse sequences of the two light sources have a periodically varying time offset. This offset comes about because the repeat frequencies of the light pulse sequences of the two light sources are slightly different.
A significant disadvantage of the ASOPS technique is that light sources whose repeat frequencies amount to at least one gigahertz have to be used for generating the light pulses. Only in this way can a time resolution that is sufficient for most applications be achieved, at the same time with practicable scanning rates.
It is another disadvantage that the scanning range of the ASOPS technique is much too large for most practical applications. This large scanning range results from the principle that the time offset between consecutive light pulses always varies periodically between 0 and the full time interval between the light pulses of one of the light pulse sequences. If, for example, the repeat frequency of the light pulse sequences is 100 MHz, the time offset of the light pulse sequences automatically varies between 0 and 10 ns. A scanning range of 10 ns, however, is not needed in practice. For most applications, for example for time-resolve spectroscopy, a variable time offset of a few 10 ps is completely sufficient, because of the time scale of the dynamics being investigated. As a result, in the ASOPS technique, no useful measurement data can be obtained during the major portion of the measurement time (more than 90%).